Hi, everyone! New member to fishlore here. Two days ago we set up a 10 gallon freshwater tank with a preset heater and Aqueon filter (it was an aquarium kit), a couple fake aquarium plants, and a tank decor barrel. We got some water, gravel, and another fake aquarium plant from a family member's established tank and added those in as well and left it for 24 hours. Yesterday, we got three ghost shrimp and put them in and they seemed to be doing well. Then purchased an API liquid master test kit and tested ammonia & nitrites with results of 0 ppm before going to get 5 neon tetras and 2 catfish today (Before acclimating the new fish & putting them in the tank we used a bottle of Tetra SafeStart because we were worried the tank hadn't cycled in the mere two days we had it, of course. Also tested ammonia and nitrites again before putting the fish in and after the Tetra SafeStart sat for an hour- results of 0 ppm again for both so we figured it was ready for fish.) After we put the new fish in, it took no more than an hour before the tetras started surfacing (we assumed for oxygen but knew it wasn't normal), so we ran to the store and purchased a bubbler and set that up in the tank thinking they needed more oxygenated water. We had to leave to meet family for dinner, and came home to the very sad and disappointing sight of all five of our tetras floating... From the time we put them in the tank to the time we came home to dead fish was a total of no more than 4 hours. We scooped them out. The two catfish & three shrimp are still alive and doing well, it seems (now 3 hours after being home from dinner). Now for the question: We decided to test the water again and this time tested ammonia, nitrites, AND nitrates, and all three read 0 ppm. We are very confused. We have redone the tests 3 times and made sure to follow the directions for each closely and REALLY shake up the #2 nitrate bottle before using it. How are the nitrates 0? Also, any idea why our tetras died? TIA!!